Source measure units (SMU) are used to make precision measurements in many fields, including the testing of semiconductor products. For example, U.S. Pat. No. 5,039,934 describes one such device and range-changing in such a device is described in U.S. Pat. No. 5,144,154, both of which are incorporated herein by reference in their entireties. Typical SMU designs include a voltage or current source with integrated voltage and current measurement capabilities. A device under test (DUT) is coupled to the SMU and is then stimulated with either the voltage or current source.
There are several ways in which the current through a DUT may be measured. For example, a shunt ammeter, may be used to simply sense the voltage across a resistor RS. RS must be kept small to not cause a large burden voltage to the input signal. A low noise gain stage is required to amplify the burden voltage so it can be measured.
A feedback ammeter uses a high gain op-amp to pull the input circuit through the resistor RS. The op-amp keeps the burden voltage low because of its high dc gain (typically greater than 1 million). This allows RS to be larger allowing the output signal to be larger. However, the op-amps high gain begins to roll off at relatively low frequencies. This causes the burden voltage to increase at higher frequencies as well. If the input is capacitive, it can cause the feedback ammeter to ring or even oscillate. It would be desirable to provide improved ammeter configurations that address these problems.